As disclosed in Patent Reference 1, a white light-emitting device comprising in combination a GaN-based blue light-emitting diode and a phosphor has been noted as a light source for display or lighting system by making the use of its characteristics of small power consumption and prolonged life. Referring to this light emitting device, the phosphor incorporated therein absorbs visible light in the blue range emitted by the GaN-based blue light-emitting diode to emit yellow light, and the blue light from the diode which has not been absorbed by the phosphor and the yellow light emitted by the phosphor are then mixed with each other to attain emission of white light.
As the phosphor there has been typically known a phosphor comprising a yttrium-aluminum composite oxide (Y3Al5O12) as a host crystal and cerium (Ce) incorporated as an activator element in said host crystal. Further, it has been known that the tone of light emitted by this phosphor can be adjusted by replacing some of yttrium (Y) atoms by gadolinium (Gd) or the like or replacing some of aluminum (Al) atoms by gallium (Ga) or the like (Non-patent Reference 1). However, there were problems that in order to produce efficiently this phosphor as a single phase, the material must be calcined at extremely high temperature that makes it difficult to produce the phosphor, and a phosphor which is uniform in emission intensity, chromaticity, particle diameter, etc. can be difficultly produced.
Also, a light emitting device comprising a blue light-emitting diode and a yellow light-emitting phosphor in combination was disadvantageous in that the emission of light in the range of from bluish green to green is short, giving deteriorated color rendering. In order to improve color rendering, a method has been proposed which comprises combining a blue light-emitting diode with a green phosphor and a red phosphor, and Non-patent Reference 2, for example, discloses a white LED comprising in combination a blue light-emitting diode, a green phosphor SrGa2S4:Eu2+ and a red phosphor ZnCdS:Ag, Cl. However, the phosphor used was disadvantageous in that it is a sulfide, can be difficultly produced and lacks stability in use.
On the other hand, as a phosphor which is produced at a relatively low calcining temperature and thus can be relatively easily produced there is disclosed a Ce-activated calcium-scandium-silicon composite oxide (Ca3Sc2Si3O12) in Patent Reference 2. This phosphor contains calcium oxides and silicon oxides that form a low-melting compound when calcined, and a calcined powder is extremely firmly sintered although the calcining temperature can be lowered. Further, this phosphor had a high emission intensity, and it was difficult to obtain a phosphor having particle diameters which are as uniform as about 1 μm to 20 μm.
On the other hand, a phosphor comprising thulium (Tm) incorporated in a scandate of alkaline earth metal having the same CaFe2O4 structure as that of the desired phosphor of the present invention is disclosed in Patent Reference 3. However, this phosphor shows emission of light having a narrow half width based on 4f-4f transition when excited with electron ray and is quite different in mechanism from that of cerium-derived emission from the phosphor of the present invention, i.e., emission of light having a wide half width based on 4f-5d transition. Further, this thulium-containing phosphor is a material which does not emit light when irradiated with ultraviolet ray or visible light, and it is thus not easy to anticipate and produce the phosphor of the present invention from the presence of this phosphor.
Moreover, phosphors comprising cerium incorporated in a strontium yttrate (SrY2O4), which, too, each are a crystal having CaFe2O4 structure, are disclosed in Non-patent Reference 3 and Non-patent Reference 4, but these phosphors do not show emission efficiently at room temperature. Further, a phosphor comprising cerium incorporated in a strontium thioyttrate (SrY2S4), which, too, is a crystal having CaFe2O4 structure, is disclosed in Non-patent Reference 5, but this phosphor was a sulfide and thus was practically disadvantageous in long-range stability, producibility, etc.    Patent Reference 1: JP-A-10-242513    Patent Reference 2: JP-A-2003-064358    Patent Reference 3: JP-A-6-100860    Non-patent Reference 1: Preprints of 264th Meeting, Phosphor Research Society, pp. 5-14    Non-patent Reference 2: Journal of The Electrochemical Society, Vol. 150 (2003), pp. H57-H60    Non-patent Reference 3: The Journal of Chemical Physics, vol. 47, pp. 5139-5145 (1967)    Non-patent Reference 4: Journal of Luminescence, Vol. 102-103, pp. 635-637 (2003)    Non-patent Reference 5: Journal of The Electrochemical Society, Vol. 139, pp. 2347-2352 (1992)